CPD Programme
> As with any gas, the moving molecules
Dry bulb thermometer
of water vapour in the air exert a pressure.
This vapour pressure, p
V
(measured in p
a
or
mbar), is related to the mass of water vapour
in the air – the more molecules present, the
higher the vapour pressure. If molecules of
water vapour were to be continually added
to a volume of static air (for example, from
a room humidifier) then at some point the
air would not be able to support any more
molecules – the air is said to be saturated at
Wetted wick Wet bulb thermometer
that point and any further water molecules
added will simply condense out. The saturated
Water reservoir
vapour pressure, p
s
, will have been reached
Freely rotating handle
– the actual value of this is related to the dry
bulb temperature of the air; air with a higher
dry bulb temperature will have a greater Figure 1 – Sling psychrometer
potential to hold water vapour, and so will
have a higher saturated vapour pressure. The produce similar values and are practically the thermometer’s bulb, so reducing the
vapour pressure of water vapour in humid interchangeable for HVAC calculations. temperature as read from the stem of the
air compared, as a percentage, with the Specific Enthalpy, h (kJ / kg
da
), is the heat thermometer. The fewer molecules of water
saturated vapour pressure of air at the same content of humid air per kg dry air – it is vapour in the air (and hence the lower the
temperature gives the relative humidity, Φ relative to the enthalpy dry air at 0°C. Air with air vapour pressure), the more molecules of
(%) and Φ = p
v
/ p
s
x 100%. higher temperature and / or with moisture water will be able to evaporate into it from
The partial pressure of the water vapour will have a positive specific enthalpy (more the thermometer’s wetted surface bulb and
adds to the partial pressures of the other commonly referred to as simply ‘enthalpy’). so the lower the wet bulb temperature.
gases in the air, to produce the atmospheric The range of conditions likely in HVAC The difference in the value of the wet bulb
pressure, p
a
, so giving p
a
= p
da
+ p
S
(this enthalpy can be estimated from h = (1.005 temperature compared with the air’s dry bulb
relationship is known as Dalton’s Law) – the θ) + g (1.89 θ + 2501); however, more precise temperature, (θ – θ') (K), is known as the wet
standard value of atmospheric pressure is results may be obtained using relationships bulb depression, (wet bulb is always less than
taken as 1013.25mbar (101.325 kPa). The is CIBSE Guide C 2007. or equal to dry bulb temperature). Dry bulb
atmospheric pressure will alter with both The thermodynamic wet bulb temperature, temperature and wet bulb depression tables
weather and location and so the relative effect θ' (°C), is a useful means of determining the (or calculations) can be used to determine
of the vapour pressure will also change. moisture content of the air at a particular dry the air moisture content. The recording
The mass of the water vapour in 1 kg of bulb temperature. The wet bulb temperature of the wet bulb temperature is commonly
dry air is termed the moisture content g is the temperature that a sample of air would undertaken in two ways – the screen wet
(kg/kg
da
) – this is really a ‘vapour’ content reach if it had a fine spray of water added bulb, θ'
sc
– and the much more useful sling
and information from the USA will refer to it (the water being at the air dry bulb wet bulb θ'
sl
. The two values are likely to vary
to this as humidity ratio. To avoid errors temperature) without any external heat being slightly from each other. The screen wet bulb
in writing down small numbers associated added or taken away (this is known as an is recorded by a static wet bulb thermometer
with moisture content, the units of g / kg
da
are adiabatic process). Some of the sprayed water placed in air (frequently outdoors in a
frequently used in place of kg / kg
da
(1g/kg
da
will evaporate into the air – the heat to cause Stevenson’s Screen), and the sling wet bulb
= 0.001kg / kg
da
). that evaporation being taken from the air is measured where there is air movement
As an example of using the relationship itself and so the air dry bulb temperature will across the thermometer’s bulb (ensuring that
pV = mRT, the moisture content, g, of drop until the point where the air is saturated any evaporated water is carried away from
a mixture of dry air and water vapour may (with water vapour). At this point the dry bulb the bulb and so does not locally increase the
be found from a knowledge of the partial temperature = wet bulb temperature, and the vapour pressure). These measurements may
pressures in the air. So: air is said to have reached its dew point: if the be undertaken in a number of ways including
mass water vapour / mass of associated air dry bulb temperature is then reduced, or the Sling Psychrometer (see Figure 1) and the
dry air = m
s
/m
a
and since more water vapour is added to the air, then Aspirated Hygrometer.
pV = mRT, then m = pV / RT, and so (p
s
V / R
s
condensation will occur.
T) / (p
a
V / R
a
T) = (Ra / R
s
) x (p
s
/ p
da
) = 287 / 461 Practically, the wet bulb temperature is Obtaining the properties
p
s
/ ( p
a
- p
s
) = 0.622 p
s
/ ( p
a
- p
s
). measured with a thermometer with a wetted CIBSE Guide C (2007) has lists of the values
By comparing the moisture content material wrapped (or more frequently a of all the properties of air as described above
in humid air as a percentage, with the wick fed from a reservoir of distilled water) for various dry bulb temperatures. The guide
moisture content of saturated air (at the same around the thermometer’s bulb at the same also includes sets of equations that will allow
temperature) the percentage saturation, null, temperature as the air dry bulb temperature. determination of values for other gases that
may be determined where null = g / g
s
x 100 per The evaporation (or drying) of the water may be used in spreadsheets or computer
cent. The numerical values of percentage from the wick into the surrounding air programmes to the Characteristic Equation may
saturation, null, and relative humidity, Φ, needs energy, and this is mainly taken from be used in conjunction with the Gas Laws.
56 CIBSE Journal April 2009 www.cibsejournal.com
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